In the following, inoperability is not necessarily to be understood as being a complete malfunction of the conduit system checked; instead, any state is so understood in which the conduit system is no longer completely operable.
U.S. Pat. No. 4,967,717 discloses mounting a temperature sensor in a return conduit for checking the operability of an exhaust-gas return conduit system. When an exhaust-gas return valve is opened, hot exhaust gas brushes over a temperature sensor whereupon the sensor is warmed. The sensor cools down again when the exhaust-gas flow is interrupted by closing the valve. If the valve is again driven so that it opens but no warming of the sensor is determined, this shows that either the valve no longer opens or the conduit system is obstructed or there is a leak forward of the location at which the sensor is mounted. The temperature of the sensor is not only dependent upon the temperature of the exhaust gas but also on the ambient temperature. For this reason, the arrangement presented in U.S. Pat. No. 4,967,717 includes a second temperature sensor for detecting the temperature of the ambient air. A threshold temperature is modified with the aid of the ambient temperature measured in this way and the temperature of the first temperature sensor is compared to the modified threshold temperature. If the temperature of the first sensor remains below the threshold temperature, then the exhaust-gas return conduit system is determined to be no longer operational.
U.S. Pat. No. 4,962,744 describes a method and an arrangement for checking the operability of the conduit system of a tank-venting system with the aid of a temperature sensor. The temperature sensor is mounted in an adsorption filter within the conduit system. If the system is operational, then the adsorption filter must adsorb fuel vapor when certain operating conditions are present; and, desorption must occur for other operating conditions. The adsorption is related to a temperature increase; whereas, the desorption leads to a reduction of the temperature of the sensor. The difference of the temperatures is formed as they are present for adsorption conditions and/or desorption conditions. The system is determined to be non-operational when this difference remains below a threshold value.
A disadvantage of the first-mentioned arrangement and of the method corresponding thereto is that two temperature sensors are needed. The arrangement is therefore relatively complex. A disadvantage of the second-mentioned arrangement and the method corresponding thereto is that a check is possible only for very specific operating conditions, the presence of which must be detected with special detectors. The second arrangement is therefore also complex. Furthermore, this arrangement has the disadvantage that it can be carried out only infrequently, namely, when the very specific operating conditions are present.